The present invention relates to a variable gain amplifier (VGA) particularly suited for automatic gain control systems (AGC).
Typically, automatic gain control (AGC) systems are implemented by employing a variable gain amplifier (VGA), a detector (a peak or average value detector) capable of providing an information on the signal level in the loop, a bidirectional current generator driven by the detector, and finally a capacitor for stabilizing the loop and constituting an analog memory of the system. A system of this type is schematically shown in FIG. 1.
Characterizing performance parameters of a variable gain amplifier (VGA), may be listed as follows:
Frequency band PA0 Range of variation of the gain PA0 Maximum level of the input signal PA0 Maximum level of the output signal PA0 Linearity PA0 Output offset PA0 Equivalent input noise PA0 Supply voltage requirement PA0 Frequency band: from 0 to 30 MHz PA0 Gain variation: from 4 to 80 Volt/Volt (26 dB) PA0 Input signal: from 12 mVpp to 250 mVpp (differential) PA0 Output signal: from 1.0 Vpp to 3 Vpp (differential) PA0 Linearity: better than 40 dB (Distortion&lt;1%) PA0 Output offset: less than .+-.400 mV PA0 Equivalent input noise: &lt;15 nV/sqrt(Hz) PA0 Supply voltage: from 4.5 V to 5.5 V PA0 Frequency band: from 0 to 50 MHz PA0 Gain variation: from 2 to 22 Volt/Volt (21 dB) PA0 Input signal: from 20 mVpp to 240 mVpp (differential) PA0 Output signal: maximum 1.1 Vpp (differential) PA0 Linearity: better than 40 dB (Distortion &lt;1%) PA0 Output offset: less than .+-.200 mV PA0 Equivalent input noise: &lt;15 nV/sqrt(Hz) PA0 Supply voltage: from 4.3 V to 5.5 V PA0 Frequency band: from 0 to 80 MHz PA0 Gain variation: from 2.7 to 33 Volt/Volt (22 dB) PA0 Input signal: from 20 mVpp to 240 mVpp (differential) PA0 Output signal: maximum 0.75 Vpp (differential) PA0 Linearity: better than 40 dB (Distortion&lt;1%) PA0 Output offset: less than .+-.200 mV PA0 Equivalent input noise: &lt;15 nV/sqrt(Hz) PA0 Supply voltage: from 3 V to 5.5 V
Specific requirements dictated by the particular field of application and therefore different performance parameters determine the choice among numerous-amplifier circuits, suitable to be integrated on silicon.
In order to implement an automatic gain control before a demodulation stage of the Intermediate Frequency block of a TV receiver, it is necessary to provide for a range of variation of the gain of about 60 dB. Therefore, it is often necessary to employ a chain of several amplifying stages in cascade. On the other hand, in consideration of the band characteristics of the processed TV signal, the coupling between the various stages is usually implemented in an AC mode. Therefore, the output offset that was mentioned above among other merit parameters of a VGA, loses its relevance in this case. A typical amplifier structure that is employed in integrated TV circuits for implementing a VGA is shown in FIG. 2.
If the control current: Icont=0, the gain of the stage is almost equal to the ratio 2RL/RE, where RE is the total emitter-degeneration resistance. By increasing Icont, the diodes turn on and, by adding their own impedance: 1/gm, in parallel to the modules that make up RE, modify (decrease) the gain.
Conversely, in the case of an automatic gain control implementation in a read/write channel of a hard disk, a typical set of performance parameters of a variable gain amplifier suited for this type of application may be indicated as follows:
In view of the relatively limited variation of the gain that is required (26 dB), a typical VGA structure that is customarily used for such an application, is constituted by a single variable gain stage "cell", followed by a fixed gain stage, typically having a gain of 10-20 Volt/Volt.
A variable gain stage that is commonly used in these applications is shown in FIG. 3. It utilizes a common circuit known as "double Gilbert multiplier circuit", which permits to vary in a continuous fashion the gain by controlling the balance in the upper quadrants and by approximately maintaining constant the DC operating point throughout the range of gain variation.
Such a known circuit has :he advantage of neutralizing the Miller effect of multiplication of a parasitic base/collector capacitance, representing in practice a cascode (grounded-base) configuration. This permits achievement of a broad band characteristic.
The integration of a read/write "channel" in a single chip and the increase of the data transfer speed have made even more stringent the requirements for the VGA to be employed, as may be set forth by the following values of required performance parameters:
The broadening of the frequency band that is required because of an increased data transfer speed, has made necessary the use of a further cascode stage for further decreasing the parasitic capacitance of the gain control resistance, according to the diagram shown in FIG. 4.
More recent developments of hard disk memory systems have imposed a further tightening of the design parameters of the VGA to be used in these state of the art systems.
Nowadays, hard disk memories are no longer exclusively installed in fixed installations, but increasingly often they are installed in portable (battery operated) instruments and the power consumption has assumed a critical importance. Moreover, these modern systems tend to be faster and faster.
As a consequence, read/write systems must be compatible with a relatively low supply voltage of about 3 Volt, the required frequency band may be up to about 80 MHz because of increase speed while the other parameters may remain practically unchanged. A sample specification of a VGA for this type of applications, characterized by a relatively low supply voltage, may be as follows:
VGA circuits that have been employed so far, fall short of reaching these performances. By considering, following example, the circuit of FIG. 4, it may be seen that this circuit is unable to provide a sufficiently wide dynamic range, capable of ensuring a correct operation with a 3 Volt supply.